mapaccess1_faststr

 
/home/qingfeng.yy/go/src/runtime/map_faststr.go
func mapaccess1_faststr(t *maptype, h *hmap, ky string) unsafe.Pointer {
	if raceenabled && h != nil {
		callerpc := getcallerpc()
		racereadpc(unsafe.Pointer(h), callerpc, abi.FuncPCABIInternal(mapaccess1_faststr))
	}
	if h == nil || h.count == 0 {
		return unsafe.Pointer(&zeroVal[0])
	}
	if h.flags&hashWriting != 0 {
		fatal("concurrent map read and map write")
	}
	key := stringStructOf(&ky)
	if h.B == 0 {
		// One-bucket table.
		b := (*bmap)(h.buckets)
		if key.len < 32 {
			// short key, doing lots of comparisons is ok
			for i, kptr := uintptr(0), b.keys(); i < bucketCnt; i, kptr = i+1, add(kptr, 2*goarch.PtrSize) {
				k := (*stringStruct)(kptr)
				if k.len != key.len || isEmpty(b.tophash[i]) {
					if b.tophash[i] == emptyRest {
						break
					}
					continue
				}
				if k.str == key.str || memequal(k.str, key.str, uintptr(key.len)) {
					return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*goarch.PtrSize+i*uintptr(t.ValueSize))
				}
			}
			return unsafe.Pointer(&zeroVal[0])
		}
		// long key, try not to do more comparisons than necessary
		keymaybe := uintptr(bucketCnt)
		for i, kptr := uintptr(0), b.keys(); i < bucketCnt; i, kptr = i+1, add(kptr, 2*goarch.PtrSize) {
			k := (*stringStruct)(kptr)
			if k.len != key.len || isEmpty(b.tophash[i]) {
				if b.tophash[i] == emptyRest {
					break
				}
				continue
			}
			if k.str == key.str {
				return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*goarch.PtrSize+i*uintptr(t.ValueSize))
			}
			// check first 4 bytes
			if *((*[4]byte)(key.str)) != *((*[4]byte)(k.str)) {
				continue
			}
			// check last 4 bytes
			if *((*[4]byte)(add(key.str, uintptr(key.len)-4))) != *((*[4]byte)(add(k.str, uintptr(key.len)-4))) {
				continue
			}
			if keymaybe != bucketCnt {
				// Two keys are potential matches. Use hash to distinguish them.
				goto dohash
			}
			keymaybe = i
		}
		if keymaybe != bucketCnt {
			k := (*stringStruct)(add(unsafe.Pointer(b), dataOffset+keymaybe*2*goarch.PtrSize))
			if memequal(k.str, key.str, uintptr(key.len)) {
				return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*goarch.PtrSize+keymaybe*uintptr(t.ValueSize))
			}
		}
		return unsafe.Pointer(&zeroVal[0])
	}
dohash:
	hash := t.Hasher(noescape(unsafe.Pointer(&ky)), uintptr(h.hash0))
	m := bucketMask(h.B)
	b := (*bmap)(add(h.buckets, (hash&m)*uintptr(t.BucketSize)))
	if c := h.oldbuckets; c != nil {
		if !h.sameSizeGrow() {
			// There used to be half as many buckets; mask down one more power of two.
			m >>= 1
		}
		oldb := (*bmap)(add(c, (hash&m)*uintptr(t.BucketSize)))
		if !evacuated(oldb) {
			b = oldb
		}
	}
	top := tophash(hash)
	for ; b != nil; b = b.overflow(t) {
		for i, kptr := uintptr(0), b.keys(); i < bucketCnt; i, kptr = i+1, add(kptr, 2*goarch.PtrSize) {
			k := (*stringStruct)(kptr)
			if k.len != key.len || b.tophash[i] != top {
				continue
			}
			if k.str == key.str || memequal(k.str, key.str, uintptr(key.len)) {
				return add(unsafe.Pointer(b), dataOffset+bucketCnt*2*goarch.PtrSize+i*uintptr(t.ValueSize))
			}
		}
	}
	return unsafe.Pointer(&zeroVal[0])
}